Vacuum brake boosters require a vacuum supplied by the engine for boosting the pedal force to be generated by the driver. Depending on the engine, even relatively low pedal forces allow reaching a condition where further increase of the force applied to the actuating unit is possible only by an increase in the pedal force because the vacuum brake booster has reached the maximum possible boosting force. This condition is referred to as the point of maximum boosting of the booster. Braking operations that take place in excess of the point of maximum boosting of the vacuum brake booster place high demands on the driver in terms of the pedal force to be generated. For this reason, brake systems (OHB-V) are employed at an increasing rate supporting the driver by means of an active hydraulic brake pressure build-up. Brake systems of this type generally include a motor-and-pump assembly and a hydraulic unit being controlled by an electronic control such as ESP, ABS, TCS, and similar systems, to comply with the desired brake force boosting.
When pressure is built up in the wheel brakes by means of hydraulic boosting beyond the point of maximum boosting of the vacuum brake booster, the (analogized) separating valves are closed, and the hydraulic pump delivers brake fluid from the tandem master cylinder (TMC) into the wheel brakes. In the pressure reduction phase, it is required to discharge the additionally generated pressure into the tandem master cylinder by way of the separating valves in a manner comfortable with respect to pedal feeling and braking effect. This action usually requires two additional pressure sensors at the wheel brakes, one per brake circuit, in order to determine the wheel pressure. The result is a higher requirement of components causing an increase in the system's costs.